PROCESS FOR RECOVERING IRON AND ZINC FROM STEEL- MAKING DUSTS

METHODE DE SEPARATION DU FER ET DU ZINC PRESENTS DANS LES POUSSIERES DE SIDERURGIE

English Abstract

Abstract
The object of the invention is to provide a
process for the recovery of iron and zinc from
steel making flue dust.
The process comprises subjecting the flue
dust to a carburizing step in a fluid bed in the
presence of a reducing agent and a carbon supplying
agent to break down the zinc ferrite structure and
convert the iron to iron carbide, followed by heat-
ing the residue or product of the carburizing step
to recover zinc by evaporization.
The iron carbide produced may be introduced
into a steel making furnace for the production of
steel.
An alternative procedure is the recovery of
iron carbide and zinc by magnetic separation from
the gangue after the carburizing step and before
separation of zinc from iron carbide.

Claims

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CLAIMS:
1. A process for the recovery of zinc and iron carbide
from a material containing zinc and iron which com-
prises:
(a) first subjecting the material to a carbiding
stew to convert a major portion of the iron to
iron carbide, and
(b) subsequently recovering zinc from the product
of step (a) leaving a residue containing substantially
all the iron carbide.
2. The process of Claim 1 in which in step (a)
the carburizing step is performed in a fluid
bed in the presence of a reducing agent and a
carbon supplying agent to convert the iron to
iron carbide.
3. The process of Claim 1 in which steel is made
from the dezinced iron carbide produced in
Claim 1.
4. The process of Claim 1 in which the mixture is
flue dust.
5. The process of Claim 1 in which in step (a)
iron carbide and zinc are separated from the
gangue of the product by magnetic separation
prior to recovery of zinc in step (b).
6. The process of Claim 1 in which in step (b)
zinc is recovered by distillation.
7. The process of Claim 2 in which the iron is
converted to iron carbide in a fluidized bed
with a mixture of hydrogren and a carbon con-
taining material which provides carbon for
the iron carbide.
8. The process of Claim 7 in which the carbon con-
taining material is carbon monoxide and hydrogen
is present in an amount over 60% by volume of the
carbon monoxide present.
9. The process of Claim 7 in which the ratio of hydrogen
to formed water in the reaction medium

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of the fluidized bed is maintained from
about 2.5 to 1 to about 8 to 1 and the ratio
of CO/CO2 is maintained from about 1 to 1
to about 4 to 1, the prescribed CO/CO2-hydro-
gen/H2O ratios being essentially in equili-
brium with methane.
10. The process of Claim 7 in which the carbon
containing material is solid carbon.
11. The process of Claim 7 in which the carbon
containing material is a lower alkyl hydro-
carbon gas.
12. The process of Claim 11 in which the gas is
propane.
13. The process of Claim 7 in which the tempera-
ture of the mixture is between about 1100°F
and about 1300°F.
14. A process for the recovery of iron and zinc
from flue dust which comprises:
(a) subjecting the flue dust to a carburizing
step in a fluidized bed in the presence of a
reducing agent and a carbon supplying agent to
convert the iron to iron carbide;
(b) heating the carburized mixture of step
(a) to recover the zinc by vaporization; and
(c) making steel from the product remaining
from step (b).
15. A process for the recovery of iron carbide and
a metal having a vaporization point above about
1300°F from a mixture of iron and the metal
which comprises:
(a) subjecting the mixture to a carburizing
step to convert the iron in the mixture to iron
carbide; and
(b) recovering iron carbide and said metal
from the product of step (a).

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16. The process of Claim 15 in which said metal is
a member selected from the group consisting of
antimony, cadmium, lead, tin and zinc.
17. The process of Claim 16 in which said metal is
zinc.
18. A process for the recovery of iron and zinc
from zinc ferrite, the recovered iron being
suitable for making steel without further
refining, the process comprising:
(a) breaking down the ferrite structure and
converting the iron to iron carbide by heating
the ferrite in a fluidized bed in the presence
of a reducing agent and a carbon supplying
material to a temperature not in excess of
about 750°C, and
(b) separating zinc from the residue of
step (a) by distillation to leave an iron car-
bide product which is suitable for making
steel without refining.
19. The process of Claim 18 in which the zinc
ferrite is contained in flue dust.
20. A process for the recovery of zinc and iron car-
bide from a material containing zinc and iron which
comprises:
(a) subjecting the mixture to a carbiding step
to convert the iron to iron carbide;
(b) separating the iron carbide and zinc from the
gangue of the product by magnetic separation; and
(c) recovering zinc from the product of step (b).

Description

~L ~C3 ~
Description
Process for Recovering Iron and Zinc
From S-teel Making ~usts
Technical Field
The invention relates to the metallurgy of
iron and zinc-and particularly to the recovery oE
these metals from flue dust from furnaces usecl in
the steel makiny industry.
The problem of economically recovering valu-
able metals, such as, iron and zinc from steel
making flue dusts has received attention for some
time in the industry. Part of the problem stems
from the difficulty of separating -these metals from
each other because much oE the zinc is combined
with the iron as zinc ferrite. It is desirable to
recover the iron in a condition which permits its
use in steel making furnaces without Eur-ther refin-
ing.
~ackground .~rt
U. S. Patents 1,780,323, 1,879,834; 3,849,121;
and 4,071,357 as well as German paten-t 2,212,351
(Cl-C22b) 3/21/74 all relate to -the recovery of zinc
from flue dust and similar ma-terials. ~11 of the
disclosed processes are hydrometallurgical pro-
cesses -typically including a leaching step with
ammonium carbonate. Most of these patents are dir-
ected towards the recovery of zinc and other non-
ferrous rnetals and do not recover the iron in a
form sultableEor reuse in steel making. Comple-te
recovery of zinc is not obtained. None of the
patents disclose a process for disrupting a zinc
ferrite skructure and recovering zinc and iron
-therefrom.
~ ~ !

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2--
Disclosure of Irlvention
In accordance with -the present invention the flue
dust containing iron and zinc, including zinc ferrite, is
subjected to a carburizing step in a fluid bed in the
presence of a reducing agent and a carbon supplying agent
to disrupt the zinc ferrite structure, and convert the iron
to iron carbide in accordance with the teaching of U. S.
Patent 4,053,301, followed by recovering the zinc by distilla-
tion after the carburization step. The remaining iron carbide
is in a condition without further refining for introduction
into a steel making furnace to produce steel in accordance
with the process of U. S. Patent 4,053,301.
Thus, in accordance with the present teachings, a
process is provided for the recovery of zinc and iron carbide
rom a material containing zinc and iron which comprises:
(a) first subjecting the material to a carbiding
step to convert a major portion o~ the iron to iron carbide,
and
(b) subsequently recovering zinc from the product
of step (a~ leaving a residue containing substantially all of
the iron carbide.
The process has the advantage that large recoveries
of relatively pure zinc are made and the iron carbide produced
is suitable for use in steel making furnaces. Further, it
eliminates the problem of disposing of zinc ferrite in a manner
to comply with environmental regulations.
Brief Description of the Drawing
The single drawing is a flowsheet of the process of
the invention.

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Best Mode of Carrying Out the Invention
The term "flue dust" as used herein lncludes
materials containing iron and other metals ~hich are
readily separated from iron oarbide, such as, zinc,
lead, cadmium and others normally associated with
flue dust from steel making.
Because of the fine particle size of the steel
making flue dust, it is preferable to pelletize the
starting material before the carburizing step, although
this is not required.

In the examples which follow the flue dust
feed material was first pelletized for the car-
burizing step into one-eighth inch diameter miniT
balls on a 24-inch laboratory balling disc using
conventional binders, such as bentonite and one
percent dextrose, followed by oven drying at 230F.
The invention is not restricted to the pelletizing
step.
Example 1
The feed material of typical BOF dust had the
following chemical analysis:
Material Percent
Fe 56.7
Zn 4.78
SiO2 2.47
A123 0.8
C 0.14
The oven-dried pel]ets were transferred to a
4-inch diameter fluidized-bed reac-tor where the
iron-bearing materials were converted to iron car-
bide using a temperature of 1120F and a balanced
mixture of CO~ CO2~ H2~ H20 and CH4 as a fluidizing
gas. After eight hours of treatment, the reactor
was cooled off and the treated product discharged.
Although there was some degradation of the
pellets, in general the product remained as pellet-
ized material. This product had the following

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anaLysis:
r~la terialPercell-t
Fe 64
Zn 5.76
5 C 5.11
The produe-t or resiclue from the carbide con-
vers.ion s-tep was plaeed on a -tube furnace using a
nil-rogen sweep gas to provide an i.nert atmosphere.
The furnace -tempera-ture was raised to 1800 F and
helcl for 120 minutes during WhiCIl time the zinc was
vaporizecl as Me-tallie zinc which condenses at the
cold end of the furnaee combus-tion tube.
After eooling, the residue was removed from
the furnaee and analyzed. This material had -the
following eomposition:
Material Percell-t
Fe 73
Zn 0.13
C 3.0
A macJnetic separation test was run on a por
-tion of the fluidized-bed earbide produ¢t or residue
to de-termine the feasibility of this me-thod of sep-
ara-tion of zine and iron carbide from -the cJangue.
The Eollowing results were obtained:
Distrihu-tion
Wt% Fe% Zn~ r`e% Zn,
Magnetic Fraction 80.2 72.2 4.5 $g.5 73
Nonmagnetic Frac-tion 19.8 34.2 6.3 10.5 27
Caleulated ~iead 64.7 5.0
Proeessing of,the magnetie concentrate from this
test through the zinc distilla-tion step produces a
Einal iron produc-t assaying over 80 percent iron and
:Less than 0.1 percent zinc. Such material is suit-

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,,
able for use as feed to a stee1.rma]c.ing ~Eurnace in
accordance with the procedure of U. S. Patent
4,053,301.
Example 2
~ second feecl material of BOF dus-t had the
following chemical analysis:
Material Percent
Fe 54.4
Zn 6.5
SiO2 1.2
C 0.3
The material was pelleti~ed and carbided fol-
lowiny the procedures of Example l, the carbidedproduct having the following chemical analysis:
Material Pe:rcent
Fe 66.8
Zn 8.06
SiO2 1.58
Ca 4.86
Mg 1.30
C 5.8
Recovery o:E zinc using the procedure of F,xample
l resulted in a product or residue having the fol-
lowing composition:
Material Percen-t
Fe 82.5
Zn 0.07
SiO2 2.20
Ca 5 45
Mg 1.55
C ` 2.1
The product was suitable feed ma-terial for
making steel as disclosed in U. S. Patent 4,053,301.

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¢xarnple 3
The Ee~ed material produced by roastincJ a hicfh
iron-zinc ore had the following composition. Tle
high tempera-ture roasting step hacl converted part
of the zinc ancl iron to zinc ferrite.
Material I'ercent
Fe 26.4
Zn 23.2
SiO2 5.2
C 0.8
Mg 1.3
Ca 2.2
Al 0.3
S 0.16
The ma-terial was carbided and dezinced as in
Example 1, the results being summarized in the fol-
lowing tab:Le.
%Fe %Zn oSiO2 %Pb %C
lIead Sample 26.4 23.35.2 5.5 0.8
Carbide Product 32.925.3 6.22 4.5 1.9
Dezinced Product 54.7 0.34 12.2 0.07 0.8
~ fter removal of the ganc~ue constituents, the
final dezinced product was suitable clS a steel
makincJ feed for the process of ~. S. Patent 4,053,30:L.
Example 4
Comparative -tests were made for zinc extraction
on the same feed material using the present process
and a conventional ~mmoniurn carbonate leach zinc
recovery process.
For one test, the carbidiny and dezincincJ 5te~ps
of Example 1 were used for applying the present pro-
cess to -the feed material.

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The conditions for the ammonium carbonate
leach cornparative test were as follows:
Feed: S0 cJrams
Leach Solution: 192 ml. (90 2/3N113; 80 3/2 CO2)
5 % Solids: 20~o
Time: 3 hours
Temperature: Ambient
The comparative results ob-tained are summarized
in the following table.
Feed Residue %Zn
G %Zn g %Zn Fil-trate Extrac-ted
1. Present
Process 50 ~ 0 55 . 5 30 ~ 2 35 ~ 2 Yellow & 61.7
Opaque
2~ Head 50~0 42.2 48~5 '10.8 Clear &6.2
Colorless
From the results o~ Example 4 i t is obvious
tha-t the carbiding step had altered the zine ferrite
to make -the zine available to other rec,overy pro-
eesses, such as ammonia leaehin-~.
The process is no-t limited to the recovery o~
zine from zinc ferrite,butineludes the recovery of
zinc and other metals mel-ting above 750 C which
accompany iron in ores and other materials. Other
me-tals than zinc oceurring with iron whieh are re-
eoverable by the process are antimony, cadmium,
lead and tin. These metals are all recoverable from
the iron carbide residue by distillation.
, I